Cast austenitic stainless steel

ABSTRACT

An austenitic stainless cast steel having a volume fraction of a ferrite phase of 0.1-5.0%.

TECHNICAL FIELD

The present invention relates to an austenitic stainless cast steel.

BACKGROUND ART

An austenitic stainless cast steel exhibits excellent propertiesespecially in corrosion resistance, strength, weldability and the like,and has been widely used for piping, valves and the like in chemicalplants and power plants. The austenitic stainless cast steel is formedof, for example from metallurgical viewpoint, two phases includingapproximately 10-20% of an alpha phase and approximately 90-80% of agamma phase (austenitic phase).

As for steel castings of the austenitic stainless steel, CF8C has beenknown. For example, a CF8C austenitic stainless steel casting includes:up to 0.08 percent by mass of C (carbon); up to 2.0 percent by mass ofSi (silicon); up to 1.5 percent by mass of Mn (manganese); 18.0-21.0percent by mass of Cr (chromium); 9.0-12.0 percent by mass of Ni(nickel); and up to 1.0 percent by mass of Nb (niobium).

CF8C includes approximately 12.0% of a ferrite phase. The ferrite phasecan be, for example, measured as ferrite content in the austeniticstainless steel with a known ferrite scope, or calculated using aSchaeffler diagram based on component elements, and is indicated withvolume fraction (percent (%)).

The ferrite phase is considered effective for preventing weld crackingand reducing stress corrosion cracking. However, if a ferrite phasecontent is large, for example, exposure of CF8C to high temperature fora long period of time may transform the ferrite phase into a sigma phase(σ phase), which is a compound of iron and chromium. This may lead toembrittlement of the steel casting.

Patent Document 1 discloses CF8C-Plus, which is an alloy modified fromCF8C, and describes that CF8C-Plus does not contain ferrite phase.Patent Document 1 also describes that CF8C-Plus includes: 0.05-0.15percent by mass of C; 0.2-1.0 percent by mass of Si; 0.5-10.0 percent bymass of Mn; 18.0-25.0 percent by mass of Cr; 10.0-15.0 percent by massof Ni; 0.1-1.5 percent by mass of Nb; and 0.05-0.5 percent by mass of N.

In Patent Document 1, an absence of the ferrite phase from CF8C-Plus isconsidered important for retaining the properties imparted at casting ofmaterials during a life of the component part produced from thematerials.

When CF8C is exposed to high temperature for a long period of time underusage environment, the sigma phase is precipitated to cause agingembrittlement, and thus aging ductility may become poor. Also in thecase of CF8C-Plus described in Patent Document 1, further improvementhas been demanded in oxidation resistance.

Therefore, it has been desired to provide an austenitic stainless caststeel exhibiting excellent aging ductility and oxidation resistance.

CITATION LIST Patent Literature

Patent Document 1: Japanese translation of a PCT application Kohyo No.2009-545675

SUMMARY OF INVENTION

In order to provide such an austenitic stainless cast steel, theinventions of the following items (1)-(6) are provided.

-   (1) An austenitic stainless cast steel having a volume fraction of a    ferrite phase of 0.1-5.0%.-   (2) The austenitic stainless cast steel according to item (1),    including: 0.01-0.10 percent by mass of C; 0.6-1.0 percent by mass    of Si; 2.0-2.8 percent by mass of Mn; and 0.1-0.4 percent by mass of    N.-   (3) The austenitic stainless cast steel according to item (1) or    (2), including: 18.0-24.0 percent by mass of Cr; 8.0-15.0 percent by    mass of Ni; and 0.2-0.7 percent by mass of Nb.-   (4) An austenitic stainless cast steel, wherein a volume fraction of    the ferrite phase is 0.1-5.0%, and the cast steel includes:    0.01-0.10 percent by mass of C; 0.6-1.0 percent by mass of Si;    2.0-2.8 percent by mass of Mn; 0.1-0.4 percent by mass of N;    18.0-24.0 percent by mass of Cr; 8.0-15.0 percent by mass of Ni;    0.2-0.7 percent by mass of Nb; and the balance is Fe and inevitable    impurities. (5) The austenitic stainless cast steel according to any    one of items (1)-(4), obtained by performing cooling from a    temperature range of 1,150-1,350° C. to a temperature range of    600-800° C. at a cooling rate of 30° C./min or more. (6) A valve    formed of austenitic stainless cast steel according to any one of    items (1)-(5).

The austenitic stainless cast steel of the present invention isexcellent in, for example, aging ductility, tensile strength andoxidation resistance, as will be described in Examples. Especially, theaging ductility in Examples of the present invention was approximately2.4 times as high as that in Comparative Examples. Likewise, oxidationresistance in Examples of the present invention was approximately 9.5times as high as that in Comparative Examples.

The reason that the austenitic stainless cast steel exhibits suchexcellent properties seems to be that the volume fraction of the ferritephase is 0.1-5.0%, and the contents of the components C, Si, Mn, Cr, Ni,Nb and N seem to play important roles. Hereinbelow, each component willbe described in detail.

By setting the volume fraction of the ferrite phase to 0.1-5.0%, evenwhen the cast steel is exposed to high temperature for a long period oftime, a precipitation amount of the sigma phase can be suppressed low.Since the precipitation amount of the sigma phase is low, the austeniticstainless cast steel is unlikely to be embrittled, and exhibitsexcellent aging ductility.

C has an effect of lowering a melting point and improving fluidity, i.e.castability of molten metal. In addition, it is preferable that theamount of C is low from the viewpoint of corrosion resistance, and if alarge amount is added, the corrosion resistance of the base metal isreduced. In view of these, in order to improve high-temperatureductility, an additive amount of C in the present invention is set to0.01-0.10 percent by mass.

Si serves as deoxidizing agent for molten metal, and is effective forimproving fluidity, oxidation resistance, and weldability. However, anexcessive addition will make the austenitic structure unstable, leadingto deterioration of castability, hinder workability and weldability, andpromotion of weld cracking. Therefore, an additive amount of Si in thepresent invention is set to 0.6-1.0 percent by mass.

Mn is effective as deoxidizing agent for molten metal, and enhancesfluidity during the casting to thereby improve productivity. Inaddition, it is also effective for reducing weld cracking. Since anexcessive addition will deteriorate oxidation resistance, an additiveamount of Mn in the present invention is set to 2.0-2.8 percent by mass.When Mn is in this range, the austenitic stainless cast steel exhibitingexcellent oxidation resistance can be obtained, as will be described inExamples.

N improves high-temperature strength and thermal fatigue resistance, andis a strong austenite forming element which stabilizes an austeniticmatrix. In addition, N is an element effective for grain refining. Withthis grain refining, ductility of the material which is important asstructure can be secured, and in addition, a drawback of poormachinability, which is specific in the austenitic stainless cast steel,can be improved. Especially, N renders excellent perforationmachinability to a member to be perforated for connecting parts. When Nis added in a large amount, embrittlement is promoted, while aneffective Cr amount is reduced and thus oxidation resistance isdeteriorated. Therefore, an additive amount of N in the presentinvention is set to 0.1-0.4 percent by mass.

Cr improves oxidation resistance and stabilizes the ferrite structure.In order to reliably attain this effect, the amount of Cr is set to 18.0percent by mass or more. On the other hand, an excessive addition willlower the aging ductility of the steel due to excessive precipitation ofCr carbide when the case steel is used at high temperature, and thus theupper limit of the Cr amount is set to 24.0 percent by mass.

Ni facilitates the formation of the stable austenitic matrix, stabilizesthe austenitic phase, and enhances high-temperature strength andoxidation resistance of the steel. Taking excellent castability,corrosion resistance and weldability into consideration, an additiveamount of Ni in the present invention is set to 8.0-15.0 percent bymass.

Nb binds with C to form fine carbide, and improves high-temperaturestrength. In addition, the formation of Cr carbide is suppressed, andthus oxidation resistance can be improved. In order to effectively exertthese effects, the content of 0.2% or more is required. However, when Nbis added in an excessive amount, heat cracking susceptibility is notablyenhanced, and inner quality will be deteriorated. Therefore, an additiveamount of Nb in the present invention is set to 0.2-0.7 percent by mass.

In addition, the austenitic stainless cast steel of the presentinvention can be produced by performing cooling from a temperature rangeof 1,150-1,350° C. to a temperature range of 600-800° C. at a coolingrate of 30° C./min or more. By producing the austenitic stainless caststeel of the present invention under the above-described conditions,even when the cast steel is left as-cast, excellent strength propertycan be obtained, and thus solution heat treatment can be omitted.

The produced austenitic stainless cast steel is used as, for example,materials for piping, valves and the like in chemical plants and powerplants.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing results of oxidation resistance (mm/year)examined with respect to the austenitic stainless cast steel.

DESCRIPTION OF EMBODIMENTS

Hereinbelow, embodiments of the present invention will be described withreference to the drawings.

The austenitic stainless cast steel of the present invention is formedin such a manner that the volume fraction of the ferrite phase becomes0.1-5.0%, preferably 0.5-3.0%. The austenitic stainless cast steel ofthe present invention includes C, Si, Mn, Cr, Ni, Nb, N and the like ascomponents thereof.

The contents are as follows:

C: 0.01-0.10 percent by mass, preferably 0.02-0.04 percent by mass;

Si: 0.6-1.0 percent by mass, preferably 0.7-0.9 percent by mass;

Mn: 2.0-2.8 percent by mass, preferably 2.2-2.4 percent by mass;

N: 0.1-0.4 percent by mass, preferably 0.15-0.25 percent by mass;

Cr: 18.0-24.0 percent by mass, preferably 19.5-21.5 percent by mass;

Ni: 8.0-15.0 percent by mass, preferably 10.5-12.5 percent by mass; and

Nb: 0.2-0.7 percent by mass, preferably 0.2-0.4 percent by mass.

The compositions (percent by mass) of the austenitic stainless caststeel of the present invention, and of CF8C and CF8C-Plus forcomparison, are shown in Table 1.

TABLE 1 Austenitic stainless cast steel of the CF8C- present inventionCF8C Plus Ferrite (volume 0.1-5.0 12.0 — fraction (%)) C (percent bymass) 0.01-0.10  Up to 0.08 0.05-0.15 Si (percent by mass) 0.6-1.0 Up to2.0 0.2-1.0 Mn (percent by mass) 2.0-2.8 Up to 1.5  0.5-10.0 Cr (percentby mass) 18.0-24.0 18.0-21.0 18.0-25.0 Ni (percent by mass)  8.0-15.0 9.0-12.0 10.0-15.0 Nb (percent by mass) 0.2-0.7 Up to 1.0 0.1-1.5 N(percent by mass) 0.1-0.4 — 0.05-0.5 

In the austenitic stainless cast steel of the present invention, bysetting the volume fraction of the ferrite phase to 0.1-5.0%, even whenthe cast steel is exposed to high temperature for a long period of time,the precipitation amount of the sigma phase can be suppressed low.Therefore, the austenitic stainless cast steel of the present inventionis unlikely to be embrittled, and exhibits excellent aging ductility.

In addition, the austenitic stainless cast steel of the presentinvention has a higher Mn content and a lower C content than those ofCF8C. With this configuration, the strength and oxidation resistance athigh temperature can be improved.

In addition to the components described above, the austenitic stainlesscast steel of the present invention may further include W, B, Al, Mo,Co, Ti, Zr, Cu, rare-earth element (La, Ce, Y, Pd, Nd and the like) orthe like, and the balance is Fe and inevitable impurities.

The austenitic stainless cast steel of the present invention can beproduced by melting the above-described metal components in a meltingfurnace and performing cooling from a temperature range of 1,150-1,350°C. to a temperature range of 600-800° C. at a cooling rate of 30° C./minor more. By producing the austenitic stainless cast steel of the presentinvention under the above-described conditions, even when the cast steelis left as-cast, excellent strength property can be obtained, and thussolution heat treatment can be omitted.

The produced austenitic stainless cast steel is used, for example, forpiping, valves and the like in chemical plants and power plants.

Example 1

Example of the present invention will be described. The main components(percent by mass) of the austenitic stainless cast steel of the presentinvention (Examples 1-1-1-6) and CF8C (Comparative Examples 1-1-1-5) areshown in Tables 2 and 3, respectively.

TABLE 2 Example 1-1 1-2 1-3 1-4 1-5 1-6 Ferrite (volume 0.2 0.2 0.2 0.20.2 0.2 fraction (%)) C 0.04 0.03 0.04 0.03 0.08 0.06 (percent by mass)Si 0.76 0.86 0.76 0.86 0.89 0.86 (percent by mass) Mn 2.07 2.15 2.072.15 2.07 2.12 (percent by mass) Cr 20.55 19.90 20.55 19.90 22.35 22.10(percent by mass) Ni 11.38 11.12 11.38 11.12 10.50 10.34 (percent bymass) Nb 0.27 0.26 0.27 0.26 0.29 0.32 (percent by mass) N 0.21 0.200.21 0.20 0.19 0.21 (percent by mass)

TABLE 3 Comparative Example 1-1 1-2 1-3 1-4 1-5 Ferrite (volume fraction(%)) 12.0 9.0 0 0 0 C (percent by mass) 0.03 0.03 0.017 0.08 0.06 Si(percent by mass) 0.47 0.63 0.37 0.48 0.57 Mn (percent by mass) 1.044.48 1.83 1.02 2.02 Cr (percent by mass) 19.98 19.93 19.93 19.35 19.60Ni (percent by mass) 9.92 9.45 11.63 11.49 11.55 Nb (percent by mass)0.59 0.42 0.43 0.69 0.71 N (percent by mass) 0.03 0.10 0.24 0.25 0.24

In these Examples and Comparative Examples, aging ductility (700° C.-620hours), tensile strength (900° C.), 0.2% proof stress (900° C.) andoxidation resistance (1,000° C.) were examined, and further ahigh-temperature low-cycle fatigue test (alternate triangular waves,strain rate of 0.1%/sec, 700° C., total strain of 0.5%) was performed.

It should be noted that both in Examples and Comparative Examples,casting was performed using normal static casting method. In Examples 1and 2, the cast steel was left as-cast, while in the other Examples andComparative Examples, the cast steel was subjected to SHT (solution heattreatment). Aging ductility, tensile strength, 0.2% proof stress, andoxidation resistance were examined and the results are shown in Table 4.

TABLE 4 Aging 0.2% High-temper- duc- Tensile proof Oxidation ature low-tility strength stress resistance cycle fatigue (%) (Mpa) (Mpa)(mm/year) test (times) Example 1-1 24.4 120 90 0.300 — Example 1-2 28.8125 87 0.370 6200 Example 1-3 24.0 113 91 0.066 3400 Example 1-4 29.2134 89 0.122 2420 Example 1-5 20.4 131 91 0.489 — Example 1-6 22.1 12988 0.394 — Comparative 17.2 93 70 1.278 2388 Example 1-1 Comparative 6.8101 75 3.494 — Example 1-2 Comparative 8.6 127 84 1.854 — Example 1-3Comparative 11.2 98 73 4.101 — Example 1-4 Comparative 8.2 104 77 3.124— Example 1-5

As a result, regarding aging ductility, Examples exhibited 20.4% ormore, while Comparative Examples exhibited 17.2% or less.

Regarding tensile strength, Examples exhibited 113-134 Mpa, whileComparative Examples exhibited 93-127 Mpa.

Regarding 0.2% proof stress, Examples exhibited 87-91 Mpa, whileComparative Examples exhibited 70-84 Mpa.

Regarding oxidation resistance, Examples exhibited 0.489 mm/year orless, while Comparative Examples exhibited 1.278 mm/year or more.

To sum up, though Examples and Comparative Examples were not notablydistinguishable in the 0.2% proof stress, it was found that Exampleexhibited excellent result in aging ductility, tensile strength andoxidation resistance. Especially, an average value of the agingductility in Examples was 24.8%, while an average value in ComparativeExamples was 10.4%, and thus the value in Example was approximately 2.4times as high as that in Comparative Example. Likewise, an average valueof oxidation resistance in Examples was 0.290 mm/year, while an averagevalue in Comparative Examples was 2.770 mm/year, and thus the value inExample was improved approximately 9.5 times as much as that inComparative Example.

The above-described results shows the case where the volume fraction ofthe ferrite phase of the austenitic stainless cast steel of the presentinvention was 0.2%, and it is considered that similar results will beobtained when a lower limit of the volume fraction of the ferrite phaseis set to 0.1%.

Example 2

In Example 1, the volume fraction of the ferrite phase of the austeniticstainless cast steel of the present invention was 0.2% (Examples1-1-1-6). In addition, also for a case in which the volume fraction ofthe ferrite phase is 1-3%, aging ductility, tensile strength, 0.2% proofstress and oxidation resistance were examined (Examples 2-1-2-4) underthe same condition for Example 1. The components of Examples 2-1-2-4 areshown in Table 5, and the results are shown in Table 6.

TABLE 5 Example 2-1 2-2 2-3 2-4 Ferrite (volume fraction (%)) 2 1 3 1 C(percent by mass) 0.014 0.013 0.020 0.013 Si (percent by mass) 0.67 0.720.62 0.72 Mn (percent by mass) 2.26 2.37 2.00 2.22 Cr (percent by mass)21.10 21.10 21.70 22.22 Ni (percent by mass) 11.29 11.38 12.09 11.54 Nb(percent by mass) 0.29 0.29 0.27 0.27 N (percent by mass) 0.22 0.23 0.160.23

TABLE 6 Aging Tensile 0.2% proof Oxidation ductility strength stressresistance (%) (Mpa) (Mpa) (mm/year) Example 2-1 27.0 128 89 0.006Example 2-2 24.0 123 88 0.058 Example 2-3 27.0 95 63 0.558 Example 2-420.4 137 88 0.015

As a result, an average value of aging ductility in Examples 2-1-2-4 was24.6%, and an average value of oxidation resistance was 0.159 mm/year.Like in Example 1, these values are recognized as being excellent overthe values in Comparative Example. It is considered that similar resultswill be obtained when an upper limit of the volume fraction of theferrite phase of the austenitic stainless cast steel of the presentinvention is set to 5%.

Example 3

With respect to the austenitic stainless cast steel whose Mn content wasapproximately 1.0-4.5 percent by mass, oxidation resistance (mm/year)was examined. As the austenitic stainless cast steel of the presentinvention, those with the Mn content of 2.26 percent by mass (Example3-1) and 2.33 percent by mass (Example 3-2) were used. As the austeniticstainless cast steel of Comparative Example, those with the Mn contentof 1.04 percent by mass (Comparative Example 3-1), 1.17 percent by mass(Comparative Example 3-2), 1.81 percent by mass (Comparative Example3-3), 4.37 percent by mass (Comparative Example 3-4), and 4.48 percentby mass (Comparative Example 3-5) were used. The components for theseExamples and Comparative Examples are shown in Table 7. The results areshown in Table 8 and FIG. 1.

TABLE 7 Example Comparative Example 3-1 3-2 3-1 3-2 3-3 3-4 3-5 Ferrite(volume 2 3 12 8 0.2 10 9 fraction (%)) C (percent by mass) 0.03 0.030.03 0.03 0.017 0.03 0.03 Si (percent by mass) 0.65 0.64 0.47 0.61 0.360.62 0.63 Mn (percent by mass) 2.26 2.33 1.04 1.17 1.81 4.37 4.48 Cr(percent by mass) 20.45 20.47 19.98 20.09 19.87 19.87 19.93 Ni (percentby mass) 11.35 11.33 9.92 9.92 12.49 9.35 9.45 Nb (percent by mass) 0.650.62 0.59 0.62 0.29 0.66 0.42 N (percent by mass) 0.14 0.12 0.03 0.120.20 0.10 0.10

TABLE 8 Oxidation resistance (mm/year) Example 3-1 0.5062 Example 3-20.4521 Comparative Example 3-1 1.2782 Comparative Example 3-2 2.6405Comparative Example 3-3 1.7060 Comparative Example 3-4 3.6345Comparative Example 3-5 3.4943

As can be seen in FIG. 1, in the austenitic stainless cast steel of thepresent invention where the Mn content is 2.0-2.8 percent by mass,oxidation resistance can be reduced to 1 mm/year or less.

Industrial Applicability

The present invention is applicable to the production of the austeniticstainless cast steel.

1. An austenitic stainless cast steel having a volume fraction of aferrite phase of 0.1-5.0%.
 2. The austenitic stainless cast steelaccording to claim 1, comprising: 0.01-0.10 percent by mass of C;0.6-1.0 percent by mass of Si; 2.0-2.8 percent by mass of Mn; and0.1-0.4 percent by mass of N.
 3. The austenitic stainless cast steelaccording to claim 1 or 2, comprising: 18.0-24.0 percent by mass of Cr;8.0-15.0 percent by mass of Ni; and 0.2-0.7 percent by mass of Nb.
 4. Anaustenitic stainless cast steel, comprising: a volume fraction of aferrite phase is 0.1-5.0%, and 0.1-0.10 percent by mass of C; 0.6-1.0percent by mass of Si; 2.0-2.8 percent by mass of Mn; 0.1-0.4 percent bymass of N; 18.0-24.0 percent by mass of Cr; 8.0-15.0 percent by mass ofNi; 0.2-0.7 percent by mass of Nb; and the balance is Fe and inevitableimpurities.
 5. The austenitic stainless cast steel according to claim 1,obtained by cooling a melted metal component from a temperature range of1,150-1,350° C. to a temperature range of 600-800° C. at a cooling rateof 30° C./min or more.
 6. A valve formed of comprising the austeniticstainless cast steel of claim 1.